Many metallic food containers (i.e. cans), for the distribution of food, have a metallic pull tab attached to one end for ease of opening to deliver contents of the can. Examples of such common cans include beer and soda pop cans. The metallic pull tab provides a convenient finger grasping means for opening the can. Typically these metallic pull tabs are riveted to metallic can ends or lids that are subsequently secured to a metallic can body containing the food stuff. These metallic pull tabs are produced in very large quantities on presses that are operated continuously, except for breakdowns, maintenance, tooling changes or tooling replacement. To reduce tooling (i.e. die) replacement due to wear it is conventional for the press operators to employ lubricants in the metallic pull tab making process. The metallic pull tab making process generally involves progressively feeding a strip of metal (e.g. aluminum or steel) into a series of dies, each of which performs a specific forming operation in a series of steps to produce the metallic pull tabs. The dies are known to be expensive and their removal for replacement or reconditioning increases, through lost production, the cost of pull tabs produced with a particular die. Thus it is highly desirable to maximize the number of parts produced before the dies must be reconditioned or replaced due to wear. To maximize die life and usage and reduce press downtime it is known in the metallic pull tab process art to employ a lubricant. Such a lubricant may be coated onto the metallic strip prior to the strip being fed into the presses for making pull tabs.
Conventionally two types of lubricants have been employed in the prior art process for making metallic pull tabs. One type is a lubricating substance dissolved in an organic solvent, usually a volatile organic solvent. The solution is applied to a metal strip from which pull tabs will be produced. On evaporation of the solvent, a residue of lubricant is left on the metallic strip. An example of such a prior art lubricant is butyl stearate dissolved in a solvent such as an alcohol. This type of lubricant and the pull tabs making process employing it is disadvantageous because it releases a volatile organic compound or substance (i.e. the solvent) into the atmosphere which may cause pollution problems as well as health and safety hazards. The release of volatile organic compounds into the atmosphere is of significant concern and is subject to government regulations (e.g. Clean Air Act) which increasingly restrict or eliminate the introduction of volatile organic compounds into the air. Further use of volatile solvents may be required to remove the lubricant residue from the finished product, increasing the use thereof to produce containers free of such residue. Thus the volatile organic content (VOC) of lubricants employed for making metallic pull tabs has been increasingly subject to restriction and disfavor. To overcome this problem a second type of lubricant, described in U.S. Pat. No. 5,125,212, has been produced and used in the metallic pull tab making process. This lubricant provides a very low or zero VOC and is a combination of mineral oil and butyl stearate. Essentially this second type of lubricant has replaced the volatile organic substance with a non volatile or low volatile substance, namely mineral oil, as a carrier for the butyl stearate. While the lubricant composition of mineral oil and butyl stearate provides a low VOC lubricant, mineral oil is known to mist (i.e. form very fine droplets suspended in air). Such mists coat machine parts with an oily residue that is often difficult and time consuming to remove and which traps other material (i.e. metal particles) which can cause wear problems on the machine. Non-aqueous (i.e. oil) based lubricant composition used in prior art pull tab making processes are often difficult to completely remove from the metallic pull tab during the tab making process or by washing and thus leave a residue on the pull tab. Further, residual mineral oil butyl stearate lubricant composition on the metallic pull tabs may have adverse effects on some properties of the food contents of the can, for example the foaming properties of beer and carbonated beverages such as soda pop, when transferred to the underside of the metallic pull tabbed lid or end for the can. It is also known that oil based lubricants can present fire hazards and disposal problems.
It is known for metallic pull tabs made in prior art pull tab making processes using prior art lubricants to leave a small residual amount of lubricant on the pull tab. Typically lids or ends, with these pull tabs attached, are stacked one upon another with the pull tab (top) side of the lid contacting the opposite (bottom) side of the lid above it. This stacking causes transfer of some of the residual lubricant from the pull tab on one lid or end to the bottom side of the lid or end above it in the stack. The lid or end having a small amount on the underside is then attached to (i.e. used to close) a metallic can containing food stuff and the food stuff can then contact the lubricant on the lid. Typically the lubricant is composed of food safe materials (i.e. materials that are safe for and approved for food contact) and does not usually present a food safety problem. However, prior art lubricants have been known to be able to adversely affect certain properties of the food stuff contents of the can. One example of such affect is the known adverse effect a lubricant may have on the foam stability of beer and carbonated beverages such as soda pop. Beers are often known and judged on the head (layer of foam) they produce when poured. Hence, it is undesirable for the beer container to adversely affect or cause collapse of the head of the beer. Therefore, can producers employ a foam collapse test which must be passed by lubricant compositions they use. In addition to passing this test, the lubricant composition must meet other requirements for food contact as well as lubrication properties. Similar problems also occur with soda pop. Any lubricant present on the interior of the lid of a beer can that would and does cause instability (i.e. reduced stability) of the head of poured beer or the foam of soda pop is undesirable. Pull tab cans used for beer and soda pop constitute a very significant percentage of the total pull tab cans produced and sold. The art therefore constantly seeks pull tab making processes using lubricants that promote: maximum tool life; maximum utilization of the pull tab making apparatus; very little or no atmospheric or other environmental pollution; low health and safety hazards; and little and preferably no adverse effects on the behavior or properties of the food contents of the can.